A bicycle is normally provided with a rear derailleur active on a sprocket assembly, which consists of a series of coaxial toothed wheels (sprockets) having different diameters and numbers of teeth coupled with the hub of the rear wheel.
The derailleur engages a transmission chain extending in a closed loop between the sprocket assembly and the crankset, moving it on toothed wheels having a different diameter and number of teeth, so as to obtain different gear ratios.
In particular, downward gearshifting is when the chain passes from a toothed wheel having a larger diameter to a toothed wheel having a smaller diameter, and upward gearshifting is when the chain moves from a toothed wheel having a smaller diameter to a toothed wheel having a larger diameter. Concerning this, it should be noted that with reference to the rear derailleur, downward gearshifting corresponds to the passage to a higher gear ratio and upward gearshifting corresponds to the passage to a lower gear ratio.
The movement in the two directions of the front derailleur is obtained through an actuation device mounted on the handlebars so as to be easy for the cyclist to maneuver.
More specifically, in a mechanical gearshift, the rear derailleur is moved between the toothed wheels of the sprocket assembly, in a first direction by a traction action exerted by a normally sheathed inextensible control cable (commonly called Bowden cable), in a second opposite direction by the release of the traction of the cable and by the elastic return action of a spring provided in the derailleur itself.
The movement of the rear derailleur is carried out according to an articulated parallelogram linkage in which the sides of the parallelogram are articulated to one another in pairs, along respective substantially parallel rotation axes, through pins. In particular, such an articulated parallelogram is formed from a support body, intended to remain fixed with respect to the frame, a so-called chain guide adapted for moving the chain between different engagement positions on the toothed wheels, and a pair of articulation arms or connecting rods (generally identified as outer connecting rod and inner connecting rod), which connect the chain guide to the support body in a movable manner.
The traction of the control cable opposes the action of an elastic spring active in the rear derailleur that tends to push the connecting rods of the derailleur towards the smallest gear of the sprocket assembly, whereas the release of the control cable frees the elastic energy of such a spring.
Therefore, the traction or release of the control cable determine respective rotations of the connecting rods of the derailleur with the consequent movement of the chain guide that makes the chain face the desired toothed wheel for precise gearshifting.
Normally, the direction in which the movement is determined by the release of the traction of the cable and by the return spring is that of downward gearshifting; vice-versa, the traction action of the control cable takes place in the direction of upward gearshifting, wherein the chain moves from a wheel of smaller diameter to a wheel of larger diameter.
In the actuation device, the control cable is actuated in traction or in release through winding and unwinding on a rotor element, commonly called cable-winding bush, the rotation of which is actuated by the cyclist with a suitable control lever, or with two control levers (a first lever for upward gearshifting and a second lever for downward gearshifting).
In order to ensure rapid, reactive and stable gearshifting it is not always suitable, during upward gearshifting, to make the transmission chain perform a single movement of size exactly coinciding with the distance that separates two sprockets.
Indeed, it is often advantageous to move the transmission chain by a greater amount than that which separates two sprockets so that it engages the preselected sprocket quicker, further moving the transmission chain to position it on the sprocket so that it maintains the engagement with it.
The cable-winding bush must therefore be rotated correspondingly during gearshifting to control the movements of the transmission chain.
Examples of mechanical actuation devices that move the transmission chain in many positions with respect to the sprockets before concluding the gearshifting are shown in documents EP2527240A1, U.S. Pat. Nos. 6,216,553, 5,676,020, 6,367,347, EP1724189 and U.S. Pat. No. 5,787,757.
The actuation device must also have a stationary hold on the cable-winding bush at the end of gearshifting.
This function is obtained with so-called indexers.
One type of indexer provides for a ball-carrying bush rotating as a unit with the cable-winding bush (set in rotation by the control lever) and equipped with a slider which engages a fastening track, equipped with stop positions, formed on an indexing bush. When the slider reaches a stop position, the ball-carrying bush and the cable-winding bush are held in the angular position reached.
In this type of indexer, the indexing bush is mounted with a small rotational play with respect to a casing that rotatably supports the ball-carrying bush and the cable-winding bush. In this way, during the initial step of upward gearshifting the indexing bush rotates with respect to the casing in an angular direction in accordance with the ball-carrying bush forcing the ball-carrying bush (and therefore the cable-winding bush) to carry out an extra rotation to take the slider into the new stop position. This extra rotation determines an extra stroke of the transmission chain during upward gearshifting.
Such extra stroke is completely recovered in the last upward gearshifting step (after the slider is engaged in a new stop position) since the indexing bush recovers the rotational play set in rotation in the opposite direction by the ball-carrying bush (which rotates in the opposite direction by means of the return of the spring of the derailleur).
The recovery of the extra upward stroke is therefore such that, at the end of upward gearshifting, the total angular excursion carried out by the ball-carrying bush (and therefore by the cable-winding bush) is equal to the angular distance that divides two stop positions on the indexing bush.
During downward gearshifting the indexing bush does not rotate, allowing the cable-winding bush to rotate by an angle coinciding with the angle that separates two stop positions on the indexing bush, positioning the transmission chain exactly at the preselected sprocket.
The Applicant has found that upward gearshifting would be further improved in terms of responsiveness and efficiency if the extra upward stroke was not completely recovered, so that the transmission chain can be positioned, when gearshifting has ended, slightly beyond the preselected sprocket and towards the sprocket of immediately greater size.
The Applicant has however found that in an indexer of the type summarily described above, the lack of complete recovery of the extra upward stroke would result in downward gearshifting that would position the transmission chain no longer at the preselected sprocket but in an off-center position with respect to it, in particular moved towards the sprocket of immediately greater diameter, with consequent slow and potentially unstable downward gearshifting.